1. Field of the Invention
The present invention relates to a switching mode power supply, and more particularly relates to a primary-side regulated pulse width modulation (PWM) controller of a switching mode power supply.
2. Description of Related Art
Switching mode power supplies (SMPS) are commonly used as primary power supplies for computers, telecom devices, and many other products. SMPS designs have many advantages over other types of power supplies, including more efficient power conversion, smaller size, and lighter weight. However, the mass component counts of SMPS designs are still a problem for power supply manufacturers and engineers. Some solutions are provided in the art without secondary-side feedback control, in order to reduce the production costs of the power supply. The applicant of the present invention filed a corresponding patent application in United States, which was titled xe2x80x9cPWM controller regulating output voltage and output current in primary sidexe2x80x9d, filed on Mar. 24, 2003 and assigned Ser. No. 10/249,214. All disclosures therein are incorporated herewith. In the corresponding application, as shown in FIG. 1, a flyback power supply without secondary-side feedback control is disclosed. This design couples the output voltage and the supply voltage via a main transformer, so that the supply voltage can be used for voltage regulation, while the output voltage is maintained within a fixed range. The output voltage can be regulated without a secondary-side feedback circuit, so the system cost can be reduced.
However, the load regulation in the invention is not considered. The precision of the load regulation deteriorates as the power supply load increases. At higher loads, the feedback signal from the auxiliary winding of the main transformer does not precisely follow the output voltage of the power supply. This happens because the rectifying diode of the auxiliary winding feedback signal causes a voltage drop proportional to the load current. FIG. 2A and FIG. 2B demonstrate the adverse effect of the rectifying diode (FIG. 1: D1) on load regulation in the corresponding patent application shown in FIG. 1. As the load current increases, so does the supply current of the auxiliary winding (ICC). This causes an increased voltage drop across the rectifying diode. Thus, as the power supply load changes, the relationship of the feedback signal (VCC) to the auxiliary winding voltage (V1) and output voltage (V2) will change. To those skilled in the art, it can be seen that because of the rectifying diode D1, the differential voltages xcex94VCL (light load) and xcex94VCH (heavy load) will not be identical. Thus the feedback signal (VCC) does not precisely follow variations to the transformer voltage. The voltage regulation. of the prior-art primary-side power supply is not consistent for all power supply loads.
Furthermore, a heavy load current will also result in a larger voltage drop across the output rectifying diode (FIG. 1: D2). Though the auxiliary winding voltage is correlated with the secondary winding voltage, the correlation weakens as the power supply load current increases. This results in a further load regulation problem.
Traditional primary-side PWM controllers can reduce production costs, but they have poor load regulation. Therefore, there is a need for an improved primary-side PWM controller that is suitable for a wider range of power supply output load levels.
A principal object of the present invention is to provide a primary-side regulated PWM controller with improved load regulation over prior-art designs. The PWM controller uses a bias current that is modulated in response to different output loads to generate different voltage drops across a detection resistor to compensate for the variation of the voltage drop across the output rectifying diode.
According to one aspect of the present invention, voltage regulation is further improved by the addition of a feedback voltage generator. A feedback voltage generator detects the flyback voltage from the auxiliary winding during each PWM cycle via the control of a sampler circuit. A sampling pulse signal generated by the sampler circuit is synchronized by a PWM signal. Cycle-by-cycle monitoring the voltage at the auxiliary winding enables feedback control to be performed more precisely. The present invention substantially improves the stability of the output voltage with respect to load changes.
According to another aspect of the present invention, voltage regulation is further improved by properly selecting the detection resistor to accurately offset the voltage drop for different types of output rectifying diode. Since the voltage drop of the detection resistor is correlated with the voltage drop of the output rectifying diode, this improves the load regulation of the output voltage under changing load conditions.
Another object of the present invention is to reduce production costs. The primary-side feedback control apparatus according to the present invention eliminates the need for secondary-side feedback control circuitry. Therefore the device count, the size of the power supply, and the production cost can be greatly reduced.
According to an aspect of the present invention, the PWM controller with primary side regulation can improve load regulation and output voltage accuracy, while reducing production costs.
It is to be understood that both the foregoing general descriptions and the following detailed descriptions are exemplary, and are intended to provide further explanation of the invention as claimed.